This invention relates to processing glass sheets and specifically refers to an improved method and apparatus for supporting self-closing tongs that grip thin glass sheets during thermal treatment. Tongs have been used for supporting glass sheets in an essentially vertical plane for thermal treatment by gripping the opposite glass sheet surfaces near their upper edges. The tongs are supported from an overhead rail which extends through thermal treatment apparatus by means of a supporting clevis mounted to a carriage that rides on the rail.
During thermal treatment, the glass sheets are conveyed through a furnace, where they are heated as uniformly as possible to an elevated temperature required for subsequent treatment. In press bending, each sheet in turn is rapidly accelerated from a position slightly upstream of the furnace exit for rapid movement to a press bending station, where the sheet is stopped as rapidly as possible, is engaged by a pair of opposing pressing molds, is disengaged and moved rapidly into a cooling station where the shaped sheet is cooled as rapidly and as uniformly as possible.
Glass gripping tongs have been provided with glass engaging elements as small as possible in order to minimize the localized obstruction to the flow of the quenching fluid. Since glass sheets are heated to substantially the softening point during thermal treatment required for tempering or for certain coating procedures, and since tongs for gripping glass sheets are provided with glass engaging elements that penetrate the heat-softened glass sheet surfaces and mar the latter, particularly when the glass sheets are relatively thin so that they require more intensive heating to arrive at a subsequent treatment station at a temperature suitable for a subsequent treatment, a need has existed for tongs with glass engaging elements that do not penetrate the opposite surfaces of the glass sheet as deeply as the prior art tongs.
Disc-shaped glass engaging elements have been used successfully on glass gripping tongs for thermal treatment of glass sheets of previous commercial thicknesses (at least 4.8 millimeter) with reduced tong marking and penetration. However, the prior art tongs needed larger diameter discs to grip thinner glass sheets properly than the discs provided previously to grip glass sheets of greater thickness. The larger sized discs interfered with the flow of quenching fluid against the glass sheet surfaces in the vicinity of the disc-shaped glass engaging elements. This interference in free flow of quenching fluid caused lower compression stresses in the glass sheets in the vicinity of the glass engaging elements than elsewhere in the cooled glass, thus leading to weak regions.
Glass sheets are usually shaped between a pair of complementary pressing members that require clearances for the glass gripping tongs during the shaping operation. The thinner the glass sheet undergoing shaping, the more critical are the requirements for maximum size of notches or openings in the glass sheet shaping members. When the clearance notches normally provided in press bending molds are too large, the control of the shape of the glass sheets is lost in the vicinity of the notches. This produces glass sheets that do not conform to the specifications established by the customer. Glass sheets that deviate from curvature to a great extent are difficult to install and may have objectionable optical properties.
Thin glass sheets lose more temperature enroute from the furnace exit to the cooling station than thick glass sheets. Hence, there is more incentive to accelerate thin glass sheets as rapidly as possible for rapid transit from the furnace to the shaping station, to stop the glass in minimum time for engagement between the press bending molds at the glass shaping station, and to accelerate the shaped glass as rapidly as possible until the entire area of the shaped glass is aligned with spaced, opposed sets of nozzles through which cooling medium is applied against the opposite surfaces of the shaped glass at the cooling station. Rapid acceleration and stopping causes freely hung tongs and their supported sheets to swing about axes extending through the thickness of the glass sheets undergoing treatment. Consequently, press bending molds of the prior art require relatively large notched portions to receive the tongs when the molds move toward one another during shaping to press bend each glass sheet in turn by engaging the latter between the closed press bending molds. Glass engaging elements for tongs have been reduced in size in the direction of glass sheet movement to reduce the size needed for the notched portions somewhat. However, further reduction in the size of the notched portions was still desirable to improve the control of the shape of glass sheets shaped by press bending, particularly those having thicknesses less than nominal thicknesses of 3/16 inch (4.8 millimeters).